Hat tip to LifeHacker for a Twitter visualisation screensaver Easter Egg. Their instructions are incomplete; their method will give a blank screen. You need to copy the contents of folder (3 items):

/Developer/Examples/Quartz\ Composer/Compositions/XML/The\ Twitterverse

to:

/Library/Screen\ Savers

Twitterverse is just one many example .qtz programs in the Developer tools and you can waste several hours trying them all. Some make creative use of an iSight camera, e.g.

/Developer/Examples/Quartz\ Composer/Compositions/Screen\ Savers/Security.qtz